Method of heat treating steel strip or the like



United States Patent O "ice 3,531,333 METHOD OF HEAT TREATING STEELSTRIP OR THE LIKE Robert R. Hill, Westlake, Ohio, assignor to Wilson,Lee

Engineering Company, Inc., Cleveland, Ohio, a corporation of Ohio NoDrawing. Filed June 24, 1968, Ser. No. 739,180 Int. Cl. C21d 1/26 US.Cl. 148--16.7 4 Claims ABSTRACT OF THE DISCLOSURE Coils of strip steelare subjected to heat treatment in a protective atmosphere of hydrogenand nitrogen. During the heat treatment, the percentage of hydrocarbongas, produced from oil on the surface of the strip, in the protectiveatmosphere is determined. The composition of the protective atmosphereis controlled during treatment so that the percentage of the hydrocarbongas relative to the percentage of hydrogen in the protective atmosphereis maintained below that at which carbon deposition will occur.

BACKGROUND OF THE INVENTION This invention relates to a method of heattreating metallic articles and more particularly to a method ofannealing steel strip in coils.

The production of steel strip commonly includes final cold rolling todesired gauge and finish. The strip as it leaves the last pass of thecold rolling mill is coiled on a mandrel of substantial diameter to forma tightly wound coil. During the cold rolling operation, the steel losesductility and in order for the desired ductility to be regained, thecoils of steel are annealed.

During annealing the coils of steel may be left in the tight coils intowhich the strip is wound as it leaves the mill and wherein eachsuccessive lap of the steel strip contacts the adjacent lap, or thetight coil may be opened into an open coil wherein a space is providedbetween successive laps. Both tight coils and open coils are annealed inbell-type furnaces in which a coil, or a stack of coils, is supportedwith its axis vertical and an inner cover placed thereover. A furnacebell, carrying heating means such as combustion tubes, is placed overthe inner cover and, in well-known manner, provides the heating meansfor raising the temperature of the strip steel to annealing value andholding it there for the desired time.

To prevent oxidation of the steel during annealing, a protectiveatmosphere, consisting primarily of hydrogen and nitrogen, is circulatedthrough the inner cover in, around and, in the case of open coils,through the coils of strip steel.

In the prior art utilization of bell-type furnaces for annealing coilsof strip steel, either in open or tight coil form, difficulties havebeen encountered in the formation of black spots which may occuranywhere on the surface of the strip in open coil annealing or adjacentthe edges of the strip in tight coil annealing. These black spots areundesirable and lower the quality of the product.

In the cold rolling of steel to form coils of strip, a light oil isapplied to the surface of the strip. Although as much of this oil aspossible is removed prior to annealing, a residue frequently remains onthe surfaces and edges of the strip and black spots may occur when acoil of such oil-contaminated strip is heated to annealing temperature.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide a method of annealing steel strip that does notPatented Sept. 29, 1970 have the disadvantages of the prior art methods.Another object is the provision of a method for annealing open or tightcoils of strip steel in a furnace wherein the possibility of depositionof carbon on the surface or edges of the steel with resultingundesirable black spots is subtantially eliminated.

This present procedure proposes introducing into the furnace containingthe coil-s of strip steel an inert protective gas comprising a mixtureof hydrogen and nitrogen and raising the temperature of the steel toabout 1150 F. (the cracking temperature of the oil but below the desiredannealing temperature for the steel). The temperature is held at thispoint until the oil on the strip has broken down or cracked forminghydrocarbon gases, largely methane (CH At this point the percentage ofmethane in the atmosphere within the furnace is determined by suitableand well known procedures. Next the temperature of the coils is raisedto the annealing temperature and held there for a desired soaking periodwhile the composition of the protective atmosphere is controlled so thatthe percentage of methane or other hydrocarbon gas relative to thepercentage of hydrogen in the enclosure will always be below that atwhich carbon deposition would occur on the steel strip.

DESCRIPTION OF PREFERRED EMBODIMENTS A description of the preferredembodiment of this invention will now be presented. A coil or stack ofcoils of strip steel is placed on the base of a conventional furnacewith the coil axes vertical. An inner cover is placed over the stack ofcoils and sealed to the base by suitable means such as a solid or liquidseal. The furnace bell is then placed over the inner cover which ispurged of air by the introduction of a protective atmosphere, forexample a mixture of hydrogen and nitrogen containing from about 4% toabout 10% hydrogen and the remainder nitrogen. Heat is generated byradiant heating tubes or other conventional means in the furnace bell toraise the temperature of the coils to between about ll2S F. and about1200 F., preferably to about 1150 F. During this heating, oil that hasremained on the surface of the coils after rolling will be crackedforming gaseous hydrocarbons, largely methane (CH which gas will becomemixed with the protective atmosphere of hydrogen and nitrogen.

By referring to the gas equilibrium curve-s it will be seen that theamount of methane (CH.,) that will remain as a gas in the presence of anatmosphere gas containing, for example, 10% hydrogen (H diminishesconsiderably as the temperature of the gases rise. Thus, in anatmosphere containing a total of 10% CH +H at 1200 F., 0.23% CH can bemaintained as a gas. Any additional amount will break down to carbonwhereas, if the temperature of the gas is increased to 1400" F., only0.069% CH will remain as a gas. With these figures in mind, it has beendetermined that by measuring the quantity of CH, in the atmosphere gasand properly controlling the composition of the furnace atmospherebefore heating it to the elevated annealing temperature, the possibilityof carbon drop-out from the gases may be completely eliminated Severalprocedures can be utilized to effect such control of the furnaceatmosphere after determining the percentage of CH, in the atmosphere andbefore heating to the elevated annealing temperature. In one method thehydrogen content of the gas is increased before heating above 1200 F. soas to maintain a quantity of hydrogen great enough to permit thequantity of CH; present to be maintained as a gas at the elevatedannealing temperature which will be used. This may be accomplished bychanging the protective atmosphere from the usual relatively inexpensivegas with 4% to hydrogen and the remainder nitrogen, to a high hydrogencontent gas such as dissociated ammonia (75% H 25% N However, the costof using such a gas would probably be prohibitive for ordinary stripannealing applications.

A second method is to determine the quantity of methane (CH in thefurnace atmosphere at the time a temperature of about 1150 F. isattained, then to increase the total fiow volume of standard 4% to 10%hydrogen-nitrogen gas through the furnace, and monitor the percentage ofmethane in the gas while maintaining the temperature of the steel atabout 1150 F. until the percentage of methane is low enough so thatfinal heating to annealing temperature, for example 1400 F., can bebrought about without carbon drop-out at that temperature.

A third control method is to increase the total atmosphere fiow throughthe furnace while holding the temperacarbon gas in said enclosure whensaid article reaches said temperature, and increasing the temperature ofsaid article to the desired maximum while controlling the composition ofsaid protective atmosphere in said enclosure during said temperatureincrease whereby the percentage of said hydrocarbon gas relative to thepercentage of hydrogen in said enclosure is maintained below that atwhich carbon drop-out 'will occur on said article.

2. A method of heat treating a steel article as defined in claim 1 inwhich said steel article is a coil of strip steel and said hydrocarbongas is methane.

3. A method of heat treating a steel article as defined in claim 1 inwhich the control of the composition of ture of the coil at about 1150F. and then, using a higher percentage of methane as an end point thanin the second method described above, increasing the hydrogen content inthe gas to 15% to 18% hydrogen during the final heating to temperaturesbetween 1150 F. and 1375 F. After reaching the final annealingtemperature (for example 1400 F.) and allowing time for the methanecontent to drop to a level where there will be no carbon drop-outdanger, use of the standard H N atmosphere gas with a hydrogen (Hcontent of between 4% and 10% could be resumed.

It can be seen from the foregoing that this invention makes it possibleto anneal in a protective atmosphere coils of strip steel having an oilresidue on the strip without the formation of the black spots frequentlyfound following prior art annealing procedures. It will also beunderstood that various modifications in the specific proceduresfollowed may be made within the scope of this invention as set forth inthe appended claims.

What is claimed is:

1. A method of heat treating a steel article having a non-gaseoushydrocarbon on its surface which includes the steps of placing saidarticle in an enclosure, passing a protective atmosphere comprising fromabout 4% to about 10% hydrogen and the remainder nitrogen through saidenclosure around said article, heating said article to a predeterminedtemperature high enough to create a hydrocarbon gas from saidnon-gaseous hydrocarbon on said article, but not high enough to causesaid hydrocarbon gas to break down into carbon and hydrogen, determiningthe percentage of said hydrosaid protective atmosphere is elfected bydetermining the percentage of methane in the atmosphere when the articleattains a temperature of about 1150 F., monitoring the percentage ofmethane in the atmosphere while increasing the fioW volume of saidprotective atmosphere and maintaining the temperatureof the steelarticle at about '1150 F. until the percentage of methane in saidatmosphere is below the level at which methane will break down intocarbon and hydrogen upon reaching a predetermined temperature, and thenincreasing the temperature of the article to said predeterminedtemperature.

4. A method of heat treating a steel article as defined in claim 1 inwhich said control of the composition of said protective atmosphere iseffected by increasing the hydrogen content of said atmosphere tobetween about 15% and 18% hydrogen while raising the temperature of thearticle from about 1150 F. to about 1400 F., and, after the methanecontent of the atmosphere has dropped to a level at which carbon willnot drop out at the final heat treating temperature with a hydrogencontent of from 4% to 10%, reducing the hydrogen content of saidatmosphere to between about 4% to about 10%.

References Cited UNITED STATES PATENTS 2,152,154 3/1939 Robiette 148-2032,177,031 10/1939 Tonner 148-16.7 2,504,808 4/1950 Dailey 14816.73,167,459 1/l965 Daguier 148--13.1

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R.

